Conventionally, noble metal powders such as those of silver, palladium, platinum, and gold, or base metal powders such as those of nickel, cobalt, iron, molybdenum, and tungsten, are used in conductive pastes as electrical materials, and in particular, in internal electrodes of multilayered ceramic capacitors. Generally, a multilayer ceramic capacitor is made by alternately laminating ceramic dielectric layers and metallic layers which are used as internal electrodes, and by forming external electrodes which are connected to the metallic layer of internal electrodes on two outside faces of the ceramic dielectric layers. As a ceramic for forming a dielectric layer, materials which contain a high dielectric constant material as a main component such as barium titanate, strontium titanate, and yttrium oxide, can be used.
On the other hand, the noble metal powders or the base metal powders mentioned above may be used to form the internal electrodes; however, since more inexpensive electrical materials are required recently, multilayer ceramic capacitors in which the base metal powders are used have been greatly developed, and in particular, nickel powder has been mainly developed.
As a general production method for a multilayer ceramic capacitor, a dielectric powder such as one of barium titanate is mixed with an organic binder so as to be suspended, and then the suspension is formed into a sheet by a doctor blade method to form a dielectric green sheet. On the other hand, a metal powder for an internal electrode is mixed with organic compounds such as an organic solvent, plasticizer, or organic binder to form a metal powder paste, and this paste is coated on the green sheet mentioned above by screen printing. Next, they are dried, laminated, and pressed, and then the organic component is removed by heat treatment, and they are sintered at about 1300° C. or at more than 1300° C. Subsequently, outer electrodes are baked on both sides of the dielectric ceramic layer to obtain a multilayer ceramic capacitor.
In the production method of the multilayer ceramic capacitor described above, organic components are vaporized and driven off by heating after the metal paste is printed, laminated and pressed on the dielectric green sheet, and the heating treatment is ordinarily performed at 250 to 400° C. in air. In this way, since the heating treatment is performed under an oxidizing atmosphere, the metal powder is oxidized, and therefore, the volume of the metal powder is increased. In addition, in recent years, evaporating to remove the organic component due to the above heating is often carried out in a reducing atmosphere, and in this case, the metal powder is reduced and shrinks. Furthermore, the metal powder is sintered at a higher temperature, after the heating for removing the organic component, and this sintering is carried out in a reducing atmosphere such as a hydrogen gas atmosphere. Thus, the volume of the metal powder is reduced.
As a result, in the process for production of multilayer ceramic capacitors, volume changes occur due to expansion or shrinkage of the metal powder by oxidizing and reducing reactions. In addition, in a dielectric, volume changes also occur by sintering, and since different materials, which are the dielectric and the metal powder, are simultaneously sintered, sintering behaviors such as volume changes due to expansion and shrinkage of each material in sintering, etc., differ. Therefore, deformation may easily occur in the metal paste layer, and as a result, breaking of the layer structure, which is called delamination, such as cracking or exfoliation, may occur.
Specifically, sintering of the dielectric which, for example, comprises barium titanate as a main component, starts at 1000° C. or more, and generally at 1200 to 1300° C., whereas in contrast, sintering of the metal powder used for the inside electrode starts at a lower temperature, for example, at generally 400 to 500° C. in the case of the nickel powder. Delamination is generated by the difference in the sintering start temperatures in the sintering behavior.
Many techniques have been suggested to overcome the delamination problem. For example, Patent Publication 1 discloses nickel powder having a tap density of more than a certain limit value versus a certain particle diameter. In addition, Patent Publication 2 discloses nickel superfine powder having an average particle diameter of 0.2 to 0.5 μm and an abundance ratio of coarse grains which is twice the average particle diameter of 0.1% or less at a number standard. The nickel fine powder disclosed in the Patent Publication 1 was developed in order to prevent the generation of internal defects such as cracking or peeling when it is used as an inside electrode of a multilayer ceramic capacitor, etc. In addition, the nickel superfine powder disclosed in the Patent Publication 2 was developed in order to prevent the generation of short-circuits, cracking, or peeling in an inside electrode of a multilayer ceramic capacitor.
Patent Publication 1 is Japanese Unexamined Patent Application Publication No. Hei 08-246001. Patent Publication 2 is Japanese Unexamined Patent Application Publication No. 2002-252139.